Nonlinearity and sharp transmission spectra of random 1D nonlinear layered structures are combined together to produce photon pairs with extremely narrow spectral bandwidths. Indistinguishable photons in a pair are nearly unentangled. Also two-photon states with coincident frequencies can be conveniently generated in these structures if photon pairs generated into a certain range of emission angles are superposed. If two photons are emitted into two different resonant peaks, the ratio of their spectral bandwidths may differ considerably from one and two photons remain nearly unentangled.
@article{arxiv.0908.3583,
title = {Photon-pair generation in random nonlinear layered structures},
author = {Jan Perina and Marco Centini and Concita Sibilia and Mario Bertolotti},
journal= {arXiv preprint arXiv:0908.3583},
year = {2015}
}